Not applicable.
The present invention relates to paper and board machine air impingement methods and apparatus for compensating for the curling tendency of a paper or board web.
As known in the prior art, multi-cylinder dryer units of a paper machine employ twin-wire draw and/or single-wire draw. In twin-wire draw, the drying cylinder groups comprise two wires which press the web one from above and the other from below against heated cylinder surfaces. Between the drying cylinder rows, generally horizontal rows, there are free and unsupported draws, in which connection the web is susceptible to fluttering, which may cause web breaks, especially when the web is still moist and therefore weak. For this reason, single wire draw has recently been adopted and applied in the dryer unit in practice without an exception, each drying cylinder group of the single wire draw including only one drying wire, on support of which the web runs through the entire group such that, on the drying cylinders, the drying wire presses the web against heated cylinder surfaces, and on the reversing cylinders or rolls between the drying cylinders, the web runs on the outer surface of the drying wire. Typically, the dryer unit of a paper machine comprises 20-30 drying cylinders and reversing cylinders, in which connection a multi-cylinder dryer has 5-8 wire groups and the groups located at the upstream end of the dryer unit are normally shorter than the groups at the downstream end thereof.
In so-called normal single-wire draw groups of the prior art, the heated drying cylinders are located in an upper row and the reversing cylinders are located in lower rows, which rows are commonly horizontal and parallel to one another. The applicant""s FI patent 54627 (corresponding U.S. Pat. No. 4,202,113) proposes placing successively above-mentioned normal single-wire groups and so-called inverted single-wire groups, in which heated drying cylinders are located in a lower row and reversing suction cylinders or rolls are located in an upper row with the main purpose of drying the web symmetrically on both sides thereof. Beloit Corp. have also put forward some proposals for dryer units comprising normal and inverted cylinder groups, in respect of which reference is made to international application publications WO 88/06204 and WO 88/06205 and to U.S. Pat. No. 4,934,067, which proposes inverted groups for a dryer unit for control of curl. According to U.S. Pat. No. 5,269,074 (Beloit Corp.), a long dryer unit applying single-wire draw is followed by a short dryer unit applying twin-wire draw with the purpose of controlling curl of the web.
The use of moist steam for straightening curl has already been known in the art since the 1970""s and the 1980""s, as appears in U.S. Pat. No. 3,948,721 (Vinheim Karl) or in U.S. Pat. No. 5,557,860 (Voith) and in public FI patent application 821431, which teaches passing the web through a steam treatment station in order to straighten curl. Recently, dryer units provided with single-wire draw have become common in which the upper or lower cylinders are steam-heated drying cylinders, the web coming into direct contact with said cylinders while being pressed by the drying wire, and in which the lower or upper cylinders are cylinders provided with internal suction, for example, Metso Paper, Inc.""s so-called VAC-ROLL(trademark) cylinders in which a vacuum effect is directed through the perforated shell of the cylinders from the interior space of the reversing cylinder to the grooves extending around the shell of the cylinder. Said vacuum effect serves to maintain the web in contact with the drying wire when the web comes to the side of the outside curve on the reversing cylinders. At the same time, the transverse shrinkage of the web is sought to be prevented while drying progresses.
In paper and board machines, the reeling of the web is usually sought to be carried out when the web is as cold as possible, and in order to achieve this aim, it is prior known that a cooling cylinder is used at the end of the dryer unit. In accordance with the commonly known state of the art, the cooling of the web has the following effects:
the relaxation time of the web can be shortened, which leads to smaller differences of stress in the web before the next process stage (e.g. calendering or reeling) as compared with a situation that the web is passed forwards at a higher temperature,
the temperature differences themselves can be reduced by lowering temperature level, which leads to smaller differences in the elastic-plastic behaviour of the web in the next process stage or before it.
The most substantial problem associated with single-wire draw is that drying heating is directed, i.e. by convection from the surface of a heated drying cylinder, more intensely only at one surface of the web from one direction. As a result of this one-direction heating, there arises a strong tendency to curl in the web. This problem is also previously known and in order to deal with it, several different solutions have been proposed in the course of years. However, it is common to all these solutions that there remain in the web more or less internal stresses which will release in an unpredictable manner at a later stage and may cause problems as soon as in connection with finishing, such as coating and reeling, or later at the stage at which the paper product is utilized.
With respect to this complex of problems and the prior art associated with the background of the invention, reference is further made to the publications
FI 902616 describes a steam box disposed in a dryer unit for relaxation of drying stresses and thus for compensating for curl.
FI 931263 describes air impingement against a large cylinder which has a diameter greater than 2 m and which is placed inside a drying wire loop. Said publication proposes the division of air impingement into sections, in which connection each section uses hot air or superheated steam having a temperature, moisture and/or pressure which is different in each section in order to prevent transverse shrinkage of the web, to control drying of the web and to achieve a desired moisture profile for the web.
FI 950434 proposes passing a web, which has a tendency to curl because of the nonsymmetrical forward-drying of the bottom and top surfaces of the web, to finishing in which the tendencies to curl are compensated for by moistening and/or plastically working the web.
FI 951748 describes a dryer unit which applies single-wire draw for control of curl and in which the last group is inverted to allow drying on both sides.
FI 963734 proposes an arrangement for drying a coated paper web in a drying group of an after-dryer unit applying single-wire draw, the web being treated in said arrangement after that by means of a steam box in order to compensate for the tendency to curl.
FI 964830 proposes an arrangement for compensating for the curling tendency of a paper web by means of an air impingement device which is placed above a drying cylinder and by which hot moist air is blown against the web.
FI 971301 discloses an arrangement for controlling the curl of a paper web by means of a dryer unit. According to said arrangement, the necessary operations are carried out in several stages while the temperature of the web is below 85xc2x0 C. According to the publication, the curl control treatment is accomplished by means of a steam box or a moistening device.
FI 971713 proposes arranging a large-diameter air impingement drying cylinder in connection with a dryer unit which applies single-wire draw and has drying cylinders placed below and reversing cylinders placed above, which air impingement drying cylinder is placed inside a drying wire loop and on top or in the vicinity of which cylinder, at both sides, heated smaller-diameter cylinders are placed, whereby, when the web is supported by the drying wire over the entire length of the dryer unit, uneven transverse shrinkage of the web can be prevented and avoided.
FI 972080 proposes disposing a steam box and/or a moistening device and/or an infrared dryer after a calender or, if calendering is not employed, in connection with a machine reel or in connection with the finishing process after it in order to compensate for curl of a web.
Despite numerous approaches of the prior art, it has not been possible to eliminate the curl of the web in paper or board machines and, recently, with increasing running speeds, the curling tendency has been also increased by the more and more common demand for downwardly open dryer units applying single-wire draw to be disposed in paper or board machines in order that the paper or board machine might be placed in a smaller, i.e. lower hall space and that, at the same time, the serviceability of the dryer unit might be improved and the contamination problems kept small. Indeed, a substantial problem with the manufacture of paper and board is still that the control of the profile ability of the web is slow, and different elongation streaks, waves or curls arise because of drying stresses, and that paper or board subjected to unequal-sided drying, in particular thin paper grades, such as different directory papers, exhibits very intense wave formation and curl when they come into contact with the moisture of air after the manufacturing process.
The primary object of the present invention is to improve compensation for the curling tendency of a paper or board web and attempt to minimize drying stresses arising in the web and to bring the curling tendency of the web to the range of reversible, or structural curl behaviour, in which connection the web is as free as possible from stresses and cooled for being wound as cold as possible. One further object of the invention is also to make control of the profile ability of the web quicker and to increase drying capacity in connection with single-wire draws.
Thus, the invention is based on a new and inventive basic idea that, in order to minimize the drying stresses of a web, in at least one zone in which the web is treated with air and which extends substantially across the entire width of the web, air impingement directed at the web includes, one following after the other, at least one hot air blowing and at least one cold air blowing in which the cold air used is hall air from the machinery hall surrounding the paper or board machine, cooled hall air and/or moistened hall air. The moisture of such hall air condenses when the air comes into an environment which is warmer than the air, with the result that the web in cold air impingement is not only cooled but also moistened by the action of the blowing air because condensed moisture is condensed and/or absorbed into the web, in which connection the curl behaviour of the web changes with moisture to the range of structural, i.e. reversible curl behaviour, which is conducive to substantially compensating for the curling tendency of paper or board.
In accordance with the invention, it is advantageous that an air impingement arrangement is arranged in a hood which is located above a drying cylinder, a suction roll or an air-impingement roll, which is advantageously the last drying cylinder, suction roll or air-impingement roll of a dryer unit, and which hood is divided with a partition wall into two sections, in which connection the web is subjected in the machine direction first to a blowing with hot air and after that to a blowing with cold air. In that connection, the air treatment zone of the web comprises a first and a second area which are defined by the bipartite hood at said hood and which extend across the width of the web. In that connection, depending on the drying wire loop arrangement, air impingement can be applied either directly to the free surface of the web or to the free surface of the drying wire located on the web. As an alternative to a bipartite hood, the air impingement arrangement can comprise in accordance with the invention
Two successive hoods placed on top of two successive drying cylinders, suction rolls and/or air impingement rolls, in which connection the former hood in the machine direction is advantageously located in connection with the second last drying cylinder, suction roll or air impingement roll and blows hot air against the web and the latter hood in the machine direction is advantageously located in connection with the last drying cylinder, suction roll or air impingement roll and blows cold air against the web. In that connection, the zone for air treatment of the web is bipartite and comprises separately a first area extending across the width of the web and located at the hood blowing hot air and a second area extending across the width of the web and located at the hood blowing cold air;
A hood arranged in connection with a drying cylinder, a suction roll or an air impingement roll, which is advantageously the last drying cylinder, suction roll or air impingement roll of a dryer unit, which hood blows hot air against the web, and a blow box or an airborne drying unit extending across the web and blowing cold air against the web. In that connection, the zone for air treatment of the web is bipartite and comprises separately a first area extending across the width of the web and located at the hood blowing hot air and a second area extending across the width of the web and located at the blow box or the airborne drying unit blowing cold air.
In accordance with the embodiments of the invention regarded as preferable, it is advantageous that the temperature of the cold air blowing is xe2x89xa650xc2x0 C. For the purpose of cooling the web further before its further treatment, a cooling cylinder can be arranged to cool the web after the air treatment zone.
With respect to the benefits of the invention, it may be mentioned that
balanced drying can be achieved which minimizes the drying stresses arising in paper,
cooling of the web before calendering equalizes the temperature differences and temperature profiles appearing in it,
cooling has been found to generally have a favourable effect on the relaxation of the web,
when drying takes place by air impingement, crystallization of lignin caused by single-wire draw cylinders can be avoided and final drying can be carried out at low temperatures,
the drying capacity of single-wire draw increases substantially, even by 10-15%,
control of drying and cooling is quick and therefore the web can be profiled quickly,
when cooling cold air blowing is coupled with hot air blowing, energy can be saved,
air impingement according to the invention can be applied both in a forward-dryer section and in an after-dryer section,
because of the downwardly open structure, the air impingement arrangement according to the invention makes it possible in a paper or board machine that removal of broke and cleaning of the unit can be carried out directly from machine level and from below the hood,
when single-wire draw is provided simultaneously with the air impingement arrangement according to the invention, blowers and other auxiliary devices can be placed on the lower level which becomes free or, especially in connection with new machines, the basement space can be left unbuilt altogether in the area of cylinder drying,
when compared with the cooling of the web accomplished by means of cooling cylinders and on the water-jet principle, the air impingement arrangement according to the invention is
clean because no drip water problem is encountered in the invention,
advantageous because no displacements of cylinders and a reel are needed, and it also
requires little space, is economical in terms of energy and easy to operate,
the air impingement according to the invention is suitable for use both in on- and off-machine dryer sections and calenders, and can also be located in the middle of a dryer section, for example, in on-machine calendering and in intermediate calendering, and
it can be applied both to coated and to uncoated papers and boards.
With respect to other special features of the invention and to the advantages attainable by them, reference is made to the dependent claims of the accompanying set of claims.